The present invention relates to a safety arrangement for a liquefied gas fuel tank, which is adapted to release an excessive pressure in a safe way. The safety arrangement is especially suitable for use with DME fuel.
It is becoming more and more common to use alternative fuels for vehicles, mainly due to environmental considerations. Such fuels involve among others ethanol, compressed natural gas (CNG) and liquefied petroleum gas (LPG), which are all widely used in some markets. There is also a further need to find fuels that are renewable in order to minimize the dependency on fossil fuels. One new fuel that can be produced from organic waste such as lignocellulosic biomass from e.g. the forest industry, is dimethyl ether or DME. DME is a gas which will be liquefied under pressure, just like LPG.
There are several safety regulations that govern the handling and use of fuels. For liquefied gas, where the gas often is heavier than air, extra safety measures are required for gas leakage. One regulation, ECE R67, governs the use of LPG in passenger cars. The different regulations concern among other the behaviour of a fuel tank exposed to excessive pressure or excessive temperature, which may cause severe damages to the vehicle and to the surrounding, if not handled correctly. One issue for fuel tanks containing liquefied gas is how to release gas when the fuel tank is exposed to a raised pressure, e.g. caused by sun radiation heating up the fuel tank.
It is known to use pressure release valves that open at a specific pressure level, such that the excessive pressure can be released. It is further known to use safety valves having a fusible element sensitive to heat that will open above a predefined temperature. These solutions may however not be optimal for all situations. There is thus still a need for an improved safety arrangement for a liquefied gas fuel tank, especially if the density of the gaseous fuel is higher than the density of air.
It is desirable to provide an improved safety arrangement for a liquefied gas fuel tank for a vehicle.
In a safety arrangement for a liquefied gas fuel tank of a vehicle, adapted to release an excessive pressure from the fuel tank, comprising a spring-loaded pressure release valve adapted to open at a predefined pressure level, the safety arrangement further comprises a housing mounted on the filet tank and having an outer opening closed by a lid, where the pressure release valve is arranged in the housing and where the housing is provided with an outlet pipe having an outlet opening, and where the lid is adapted to melt at a predefined temperature, thereby allowing the liquefied gas to exit the pressure release valve through the outer opening of the housing when the pressure release valve is open and the lid has melted. The liquefied gas fuel may e.g. be D E.
By this first embodiment of the safety arrangement according to the invention, the safety arrangement is adapted to allow a small controlled fuel release down to the ground from the fuel tank caused by an excessive pressure, and to allow a large fuel release in another direction when the fuel tank is exposed to excessive heat, caused e.g. by a fire. The safety arrangement is provided with an outlet pipe having an outlet opening directed downwards, towards the ground. The released fuel, which will be in the form of gas, is heavier than air which means that the gas will be discharged to the ground in a controlled way, and cannot accidentally flow towards a hot item on the vehicle.
When the fuel tank is exposed to excessive heat, e.g. if there is a fire under the vehicle, the pressure in the fuel tank will rise such that the pressure release valve will open. In this case, it is of advantage to discharge the fuel in a direction not pointing downwards towards the ground, since the fuel will catch fire. When the fuel tank is exposed to excessive heat, the amount of gas or liquid that has to be released is large. At larger fuel release rates, the fuel can be both in a gas state and in a liquid state. It is thus of advantage to direct the released fuel in a direction pointing away from the fire.
In the inventive safety arrangement, the pressure release valve is arranged in a housing. The outer opening of the housing is provided with a gas tight lid which allows small amounts of gas to be directed through an outlet pipe, and where the lid will melt at a high temperature. The gas or liquid can in this case discharge through the outer opening of the housing. Due to the high pressure, the fuel will be discharged in a jet containing both gas and liquid. By letting the fuel out in a radial direction not pointing towards the fire will prevent the fuel from adding energy to the fire.
The lid is preferably made from a transparent plastic having a relatively low melting temperature. In this way, the lid will melt when the fuel tank is exposed to excessive heat, such as from a fire. A further advantage of having a transparent lid is that it is possible to see the state of the pressure release valve. In this example, the pressure release valve is provided with a one way cap which will come off when the pressure release valve opens, e.g. to release a small amount of gas. When the excessive gas has been released, the pressure release valve will close again, but the cap will remain off.